As a method for manufacturing a polyacrylonitrile (hereinafter referred to as PAN) fiber which is a carbon fiber precursor, there has been conventionally performed a method in which a spinning dope is formed into a fiber by wet spinning or dry jet spinning, the obtained fiber is subjected to first drawing, drying, and then subjected to second drawing through a steam tube or the like. The first drawing process therein is a drawing process performed subsequent to the spinning process in the above-mentioned series of processes. Since the drawing is usually performed in a bath such as in warm water, it is also called a bath drawing process. The second drawing process means a drawing process which is additionally performed when a yarn is dried once after the first drawing process. Thus, in the spinning of a PAN fiber which is a carbon fiber precursor, drawing is usually performed twice, of which the former is referred to as first drawing and the latter is referred to as second drawing.
For the purpose of reducing the cost of a carbon fiber, it is believed that the spinning speed of a PAN fiber is increased to improve productivity per unit time. Japanese Patent Laid-open Publication No. 2008-248219 discloses that stringiness is dramatically improved by blending a small amount of high molecular weight PAN with normal molecular weight PAN, thereby achieving high-speed spinning.
In the case where steam drawing using a steam tube is performed as the second drawing process, however, there are fears that increase of the spinning speed for the purpose of improving productivity of a PAN fiber leads to increase of steam leakage from the steam tube and the steam tube needs to be lengthened, which may result in increase in cost. In addition, the use of the lengthened steam tube makes it difficult for a yarn to pass through the tube. Therefore, a second drawing method other than steam drawing has been desired for high-speed spinning. One of the solutions to this is hot drawing.
However, hot drawing cannot be expected to provide the effect of plasticizing by steam such as steam drawing so that there arises a problem that the draw ratio cannot be increased. Further, our studies revealed a problem that the high-speed spinning disclosed in JP '219 would make it more difficult to perform drawing at a high draw ratio.
In hot drawing, multistage hot roll (hereinafter referred to as an HR) drawing in which a plurality of HRs are combined has been studied. Each stage, however, provides low draw ratio, thereby making it difficult to improve productivity (Japanese Patent Laid-open Publication No. 11-200141).
On the other hand, Japanese Patent Laid-open Publication No. 09-078333 discloses that in the hot drawing, a yarn is preheated with a hot roll (HR) and the preheated yarn is subjected to HR-HPL drawing (hot plate drawing) in which a hot plate (hereinafter referred to as an HPL) is arranged so that the maximum draw ratio at break is improved. However, since a contact length (HPL length) between the HPL in use and the yarn is 1 m, which is rather long, the yarn is resident on the HPL over a long period of time (approximately 1.2 seconds) and then deformed by drawing, so that the drawing may tend to become unstable. In addition, Japanese Patent Laid-open Publication No. 04-263613 also discloses hot plate drawing in Comparative Example 1, in which the effect of improving the draw ratio by an HPL is also disclosed. The HPL length is so long as 1 m, however, that the drawing tends to become unstable, and thus U %, which is an index of yarn unevenness, of the drawn yarn is increased as compared with the one obtained in normal HR-HR (HR drawing) (Comparative Example 1 in JP '613). Therefore, JP '613 proposes that hot pins are placed between HPLs and the draw ratio is shared with the hot pin portion where the drawing point is easily fixed and the HPL portion, to thereby reduce yarn unevenness. It is preferable that such yarn unevenness is reduced because continuous drawing for a long period of time can induce fuzz or yarn breakage. Although the use of hot pins can improve U %, there still arises a problem that abrasion between the hot pins and the yarn is likely to induce fuzz or yarn breakage.
Although stretchability and stainability can be improved by copolymerizing large amounts of a second component and a third component into PAN like an acrylic fiber for clothing. However, when the resulting product is used as a carbon fiber precursor, components to be lost during an oxidization and carbonization treatment increase. Therefore, not only the yield of carbon fiber decreases, but a defect is likely to generate in the carbon fiber, which may deteriorate mechanical properties in some cases.
It could therefore be helpful to provide a method of manufacturing a polyacrylonitrile fiber which is excellent in productivity with little fuzz and less yarn breakage, together with a sufficient draw ratio obtained even during high-speed hot drawing.